Analog Devices LINEAR LTM 8005 Manual page 14

LTM8005
APPLICATIONS INFORMATION
PWM Dimming Control for Brightness
There are two methods to control the LED current for
dimming using the LTM8005. One method uses the CTRL
pins to adjust the current regulated in the LEDs. A sec-
ond method uses the PWM pin to modulate the LED cur-
rent between zero and full current to achieve a precisely
programmed average current, without the possibility of
color shift that occurs at low current in LEDs. To make
PWM dimming more accurate, the switch demand cur-
rent is stored on the VC node during the quiescent phase
when PWM is low. This feature minimizes recovery time
when the PWM signal goes high. To further improve the
recovery time, a disconnect MOSFET switch has been
implemented to open the LED current path to prevent the
output capacitor from discharging during the PWM signal
low phase. The minimum PWM on or off time depends
on the choice of operating frequency set by the RT input.
For best current accuracy, the minimum recommended
PWM high time should be at least three switching cycles
(3µs for f = 1MHz).
SW
A low duty cycle PWM signal can cause excessive start-up
times if it is allowed to interrupt the soft-start sequence.
Therefore, once start-up is initiated by a PWM signal, the
LTM8005 will ignore a logical disable by the external PWM
input signal. The device will continue to soft-start with
switching and TG enabled until either the voltage at SS
reaches about 1V or the output current reaches one-fourth
of the full-scale current. At this point the device will begin
following the dimming control as designated by PWM. If
at any time an output overcurrent is detected, the internal
MOSFETs will be disabled even as SS continues to charge.
Programming the Switching Frequency
The RT frequency adjust pin allows the user to program
the switching frequency from 100kHz to 1MHz to opti-
mize efficiency/performance or external component size.
Higher frequency operation yields smaller component
size but increases switching losses and gate driving cur-
rent, and may not allow sufficiently high or low duty cycle
14
operation. Lower frequency operation gives better perfor-
mance at the cost of larger external component size. For
an appropriate RT resistor value see Table 1. An external
resistor from the RT pin to GND is required—do not leave
this pin open.
Table 1. Typical Switching Frequency vs R
Spread Spectrum Frequency Modulation
Switching regulators can be particularly troublesome
for applications where electromagnetic interference
(EMI) is a concern. To improve the EMI performance,
the LTM8005 includes a spread spectrum frequency fea-
ture. If there is a capacitor (C
triangle wave sweeping between about 1V and 2V is gen-
erated. This signal is then fed into the internal oscillator
to modulate the switching frequency between about 70%
of the base frequency and the base frequency, which is
set by the RT resistor. The modulation frequency is set by
12µA/(2 • 1V • C
are sensitive to the RAMP frequency selected with the
capacitor. 1kHz is a good starting point to optimize peak
measurements, but some fine tuning of this selection
may be necessary to get the best overall EMI results in a
particular system. Consult factory applications for more
detailed information about EMI reduction. The Typical
Performance Characteristics section contains plots that
show the LTM8005 conducted and radiated emissions
with and without Spread Spectrum enabled.
For more information www.analog.com
f (kHz)
OSC
1000
900
800
700
600
500
400
300
200
100
RAMP
). The results of EMI measurements
RAMP
Value (1% Resistor)
T
R (kΩ)
T
6.65
7.50
8.87
10.2
12.4
15.4
19.6
26.1
39.2
82.5
) at the RAMP pin, a
Rev. B